One in five U.S. gun owners who obtained a firearm in the past two years did so without a background check, according to a new national survey conducted by researchers at Northeastern University and Harvard University.

The study also found the share of gun owners who acquired firearms via private sale without background checks was significantly larger (57 percent) in states without laws regulating such purchases than in states with legislative regulations (26 percent).

The findings were published Tuesday in the journal Annals of Internal Medicine.

“This is the first direct national estimate of the proportion of gun owners who obtained firearms without a background check,” said Matthew Miller, the study’s lead author and professor of health sciences and epidemiology in Northeastern’s Bouvé College of Health Sciences.

The proportion of gun owners who had obtained firearms without background checks had previously been estimated using data from a 1994 study that asked whether respondents had obtained their most recent firearm from a federally licensed dealer. The 1994 survey was widely quoted as estimating  that 40 percent of gun transfers were conducted without background checks. However, Miller said that the 1994 study focused primarily whether respondents identified the source of their firearms as a federal firearms license and on where gun owners had obtained their firearms, but didn’t ask directly if the firearms were obtained without  background checks per se. The new Northeastern-Harvard survey asked directly—and found that 22 percent of current U.S. gun owners who acquired a gun within the past two years did so without background checks.

This is the first direct national estimate of the proportion of gun owners who obtained firearms without a background check.
—Northeastern professor Matthew Miller

While Miller noted that it is not clear how much of the apparent decline from 40 percent to 22 percent can be attributed to differences in the survey questions in 1994 and 2015, he underscored that it looks like we are closer today than we were 20 years ago to universal background checks. But Miller also cautioned that even today, millions of American adults continue to acquire guns annually without background checks. He added that while research shows the overwhelming majority of Americans favor universal background checks, more than 30 states don’t require background checks on private firearm sales.

“Our research makes the case for the adoption of laws in states that do not currently regulate private firearm transfers,” Miller said, “and it underscores the fact that we’re talking about millions of gun transfers annually that pass from one private owner to another without a formal vetting process and so without knowing whether the recipient is someone society deems a lawful possessor of firearms.”

Other survey findings:
•    Half of the firearms purchased privately within the past two years were obtained without a background check.
•    77 percent of gun owners who purchased their most recent gun from a friend or acquaintance did so without a background check.
•    45 percent of gun owners who purchased their most recent gun online did so without a background check.

The nationally representative survey, which was conducted in April 2015, included 1,613 gun owners.

The new survey comes on the heels of another study by Miller and his colleagues last year that found the estimated number of privately-owned guns in America grew by more than 70 million—to approximately 265 million—between 1994 and 2015. Half of that gun stock, the study found, is owned by only 3 percent of the population.